SuspendSepolicyTests: Use target device grep

Some vendors reported issue running the test since thier host
environment was missing the path to grep command.

Use the target device grep instead.

Bug: 381369000
Test: atest SuspendSepolicyTests
Change-Id: Id891e9bcdfe00992b98124319e821d0bbb663282
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>