Add activity-alias to TEST_ACTIVITY of higher prio

This hopefully allows our test to resolve only activities in the current
test, and not resolve to intent filters from accidentally left behind
other test APKs that might be around.  We need to keep the intents as
implicit as this is what we are testing.

Test: atest SafetyCenterActivityTest
Bug: 287065919
Relnote: N/A
Change-Id: Ife3209ab4618568d959bd0a2d6f6d0935e3bd2b5