Add a test to check unusual home activities

Framework's SystemUserHome and Shell's NullHome activities have no real
Home function and should only be used in bootable CSI products. They
should not exist or be enabled on normal products.

This test can help to find if they exist in the system without needing
to find them via Settings manually.

Bug: 149307007
Bug: 149780604
Bug: 154723738
Test: # On aosp_crosshatch with Shell's NullHome activity
      $ atest NullHomeTest
      ...
      arm64-v8a NullHomeTest
      ----------------------
      com.android.test.nullhome (1 Test)
      [1/1] com.android.test.nullhome.NullHomeTest#checkNullHome: FAILED (127ms)
      # On aosp_crosshatch without Shell's NullHome activity
      $ atest NullHomeTest
      ...
      arm64-v8a NullHomeTest
      ----------------------
      com.android.test.nullhome (1 Test)
      [1/1] com.android.test.nullhome.NullHomeTest#checkNullHome: PASSED (2ms)
Change-Id: I6f190ae33fc9e724adc0e53d07107e9a1ae0291f