[GWP-ASan] Provide runtime configuration through an env var + sysprop.

This patch introduces GWP-ASan system properties and environment
variables to control the internal sampling rates of GWP-ASan. This can
be used for:

 1. "Torture testing" the system, i.e. running it under an extremely
    high sampling rate under GWP-ASan.
 2. Increasing sampling remotely to allow further crash report
    collection of rare issues.

There are three sets of system properites:
 1. libc.debug.gwp_asan.*.system_default: Default values for native
    executables and system apps.
 2. libc.debug.gwp_asan.*.app_default: Default values for non-system
    apps, and
 3. libc.debug.gwp_asan.*.<basename/app_name>: Default values for an
    individual app or native process.

There are three variables that can be changed:
 1. The allocation sampling rate (default: 2500) - using the environment
    variable GWP_ASAN_SAMPLE_RATE or the libc.debug.gwp_asan.sample_rate.*
    system property.
 2. The process sampling rate (default: 128 for system apps/processes, 1
    for opted-in apps) - using the environment variable
    GWP_ASAN_PROCESS_SAMPLING or the libc.debug.gwp_asan.process_sampling.*
    system property,
 3. The number of slots available (default: 32) - using the environment
    variable GWP_ASAN_MAX_ALLOCS or the libc.debug.gwp_asan.max_allocs.*
    system property.

If not specified, #3 will be calculated as a ratio of the default
|2500 SampleRate : 32 slots|. So, a sample rate of "1250" (i.e. twice as
frequent sampling) will result in a doubling of the max_allocs to "64".

Bug: 219651032
Test: atest bionic-unit-tests
Change-Id: Idb40a2a4d074e01ce3c4e635ad639a91a32d570f

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