Fix the values of soong_config_variable

Correct the values of the Soong config variables
`TARGET_SUPPORTS_32_BIT_APPS`, `TARGET_SUPPORTS_64_BIT_APPS`, and
`building_gsi`.

`TARGET_SUPPORTS_32_BIT_APPS` and `TARGET_SUPPORTS_64_BIT_APPS` should
only be set to `true` or `false` after checking their values.

`building_gsi` should be set with a `default value` to prevent it from
always being empty due to never being explicitly set.

Bug: 3446751
Test: m -j TrichromeLibrary-Stub
Change-Id: Idf3bf1a47d7ae40880137e36ac1f0aa4b1b6dcc7