LockPatternUtilsTest: don't call the real TrustManagerService

As LockPatternUtilsTest is a unit test that doesn't otherwise interact
with the real system services, inject a mock TrustManager so that
LockPatternUtilsTest#testSetEnabledTrustAgents doesn't call into the
real TrustManagerService#reportEnabledTrustAgentsChanged.  This was
making this test require the ACCESS_KEYGUARD_SECURE_STORAGE permission.
This fix makes it possible to merge the utiltests version of
LockPatternUtilsTest into the coretests version.

Bug: 321737173
Ignore-AOSP-First: Conflict due to security fix ag/25939141
Test: atest FrameworksUtilTests:LockPatternUtilsTest
Change-Id: I81e25db9acbceefbc6e37352820dc0bd76d61b76